The shapefile is taken from the US Census Bureau's 2013 Cartographic Boundary Shapefiles - ZIP Code Tabulation Areas (ZCTAs). See ?zip.map for additional information. Metadata about each ZCTA is in ?zip.regions. Metadata includes State, County and Metropolitan / Micropolitan Statistical Area (MSA). Visualization functions are ?zip_choropleth and ?zip_choropleth_acs.